The invention relates to well screens and particularly to well screens having a perforated pipe base of the type commonly used in the production of oil and gas. For many years, spiral wound well screens of the type disclosed in Johnson U.S. Pat. No. 2,046,458 have been used in water wells for permitting the passage of water through the surface of the screen and into a pipe connected at the upper end thereof which carries the water to the surface. Wells for the production of water are generally of much shallower depths than those used for the production of oil and gas and there is usually very little cause to withdraw the well pipe and the attached screen once the well is completed. The situation is far different in the production of oil and gas since such wells often extend many miles below the surface of the earth. The greater depth of the wells requires that the well screens and pipes have a much greater resistance to compressive, tensile and torsional loading than is the case with water wells. It has been found that the necessary strength to resist such loading can be provided for a well screen by mounting a screen jacket member on a perforated pipe base support member. An example of such an assembly is shown in my co-pending application Ser. No. 258,360 filed Apr. 28, 1981, now U.S. Pat. No. 4,378,840 in which one end of the screen jacket is welded to the pipe base while the other end is free to move at the high temperatures present in an injection well so as to prevent damage from differential expansion of the members. When the pipe and screen jackets are used in injection wells, it is generally necessary to remove them from the well after about 6-36 months of service to replace the gravel pack which deteriorates rapidly under the punishment it receives in the alternating cycles of injecting steam and removing oil, with the temperature changing continually.
A well screen assembly which has the upper end of the screen welded to the pipe base and the lower end free, but sealed relative to the pipe base with an elastomeric ring is disclosed in Sears U.S. Pat. No. 4,167,972 assigned to a common assignee. Such a screen would preserve the integrity of the screen jacket as compared to a jacket welded at each end in a situation where the pipe base is subjected to large tensile loads during its removal from a well. However, where the sealing means comprises an elastomeric ring, the seal can deteriorate very rapidly at temperatures over about 500.degree. F., and thus would be of little value in an injection well experiencing higher temperatures. The cost of an elastomeric ring and associated construction and labor costs are also quite high.
Reference has been made to the problems which can arise when a screen is heated. Such a situation arises when advanced recovery techniques are used such as in steam injection of oil formations. These techniques are increasingly being employed in oil production in order to lower the viscosity of residual oils. Where the screen is welded to the base pipe at each end, the stresses produced in the screen would be compressive due to the increased thermal expansion of the stainless steel screen compared to that of the low carbon steel base pipe. This expansion could lead to failure of the screen by localized buckling. One primary effect of this would be opening of the screen slots causing sand to be pumped. A second primary effect would be to increase the chance that the screen wires or rods would separate and interfere with the ability to retrieve the screen. Presently used steam injection temperatures run from about 500.degree.-650.degree. F. At a temperature of 650.degree. F., the differential expansion of a stainless steel screen relative to a low carbon steel pipe base is about 0.22" per 10' length. Since a single screen is typically from 20-40' long, a differential expansion of about 0.44-0.88" would be expected in going from room temperature to 650.degree. F. In the "huff and puff" cycle type of injection operation, steam is injected for perhaps a month to heat the formation to 500.degree.-650.degree. F. and then oil is pumped for several months until its viscosity becomes too high as it cools to perhaps 300.degree. F. This constant cycling between 300.degree. F. and 650.degree. F. would subject a stainless steel screen, which was welded at both ends to a low carbon steel base, to very substantial compression forces which would tend to cause it to buckle.
In order to provide a sufficient amount of screen openings in a given formation, it is common to provide hundreds of feet of screen length by joining together a large number of 10'-40' screen sections. This produces yet another problem when the well has relatively high down-hole temperatures. For example, where a well contains 5000 feet of carbon steel pipe and 320 feet of screen and operates at 650.degree. F., it can be expected that the portion of the pipe supporting the topmost screen section will elongate as much as 14" relative to the portion of the pipe supporting the bottom section. Since the well is usually packed with gravel before it is made operational, it is easy to see that if the screen jacket segments were fixed at either or both ends to the pipe base, the gravel surrounding the pipe could exert a tremendous shearing force on the screen jacket segments, possibly shearing them free from the pipe base support as the pipe and screens moved during heating. It is further obvious that even if the segments were not sheared off, they would constantly abrade and destroy the gravel pack as they move up and down next to it upon cycling of the temperature. This abrasive action can result in a need to remove the well string and replace the gravel pack, and usually the screen jackets, much sooner than if relative movement between the gravel pack and screen jackets did not take place. One possible solution to preventing such relative movement is to pin the screen jacket to the pipe and provide packings or an elastomeric O-ring as a seal between the ends of the jacket and pipe. The pin would serve to hold the parts together during assembly of the screen into the well but, to permit relative movement when the well is operational, must then either be somehow removed after assembly or made of a material which will shear. The shearing of the pin would be caused by the upward expansion of the pipe when it is heated since the friction exerted on the screen jacket by the gravel pack would tend to prevent corresponding movement of the screen jacket. This shear pin type of connection would not prevent relative rotary motion between the screen jacket and pipe base and could thus hamper future recovery operations. For example, in recovery operations, an annular rotary cutter is usually lowered around the outside of the screen jacket to cut through the metal screen "centralizers" which are usually fixedly positioned at spaced location along the length of the screens. The centralizers have radially extending fins which are used for centering the screen in the casing as the gravel pack is inserted, but the fins must be cut before the screens can be removed. The cutting by the rotary cutter is quite simple when the screen cannot rotate but where the screen can rotate, the cutter would tend to dig into the centralizer fins and cause them to rotate the screen, thus preventing the relative movement required for cutting to take place. The use of packings or elastomeric O-rings which have to continually slide over the pipe base also presents problems since the packing material can be blown out of its intended location while the O-rings can rapidly deteriorate at high temperatures.